Enhanced spin state readout of nitrogen-vacancy centers in diamond using infrared fluorescence

Nitrogen-vacancy (NV) centers in diamond have been used in recent years for a wide range of applications, from nanoscale NMR to quantum computation. These applications depend strongly on the efficient readout of the NV center's spin state, which is currently limited. Here we suggest a method of reading the NV center's spin state, using the weak optical transition in the singlet manifold. We numerically calculate the number of photons collected from each spin state using this technique, and show that an order of magnitude enhancement in spin-readout signal-to-noise ratio is expected, making single-shot spin readout within reach. Thus, this method could lead to an order of magnitude enhancement in sensitivity for ubiquitous NV-based sensing applications, and remove a major obstacle from using NVs for quantum information processing.

Figure 1

Energy-level diagram and relevant transitions for the neutral and negatively charged NV center. Green excitation is depicted with green arrows, red fluorescence is depicted with downward red arrows, IR excitation and fluorescence are depicted with purple and red arrows, respectively, nonradiative decay is depicted with blue arrows, ionization and recombination transitions depicted with dashed black arrows.

Figure 2

SNR as a function of green excitation power and pulse duration for bulk (a), (c) and surface (b), (d) NVs, without (a), (b) and with (c), (d) time normalization. A big difference in the SNR between bulk and surface NVs is noticed due to the difference in ionization cross sections.

Figure 3

IR fluorescence spin-state readout. (a) Pulse sequence for the IR fluorescence spin readout. (b)–(e) SNR as a function of IR excitation power and pulse duration for bulk NV (b), (d) and surface (c), (e) NVs, with (d), (e) and without (b), (c) normalization. Higher absolute and normalized SNR can be theoretically achieved compared to red fluorescence readout, for both bulk and surface NVs, using strong enough excitation.

Figure 4

Schematic drawing of the suggested experimental setup and electric field energy density of the photonic crystal structure. (a) Schematic drawing of the suggested experimental setup. Green and IR lasers are focused onto the diamond sample in a photonic crystal cavity structure using a high NA objective lens, while AOMs control the pulse sequencing. Fluorescence is then collected through the same objective and directed to an SPCM after filtering the red fluorescence and the green and IR photons reflected from the diamond's surface. (b) Photonic crystal structure and electric field near-field energy density. Three holes are shifted from each side to optimize Purcell factor. (c) Electric field far-field energy level density, showing highly directed emission from the cavity.

Figure 5

Expected spin state SNR, fidelity and number of photons emitted as a function of Purcell factor under 1W excitation (inside the cavity), $1\mu s$ readout duration and an optimized delay duration $\tau =10\mathrm{ns}$. (a), (b) Spin-state readout SNR (a) and fidelity (b) for bulk (blue lined) and surface (red lines) NVs. The black line illustrates the highest SNR/fidelity possible for bulk NV using red fluorescence. (c), (d) Number of photons emitted during the singlet excitation for $m_s=0$ (black lines) and $m_s=\pm 1$ (green lines) for bulk (c) and surface (d) NVs.

Figure 6

Expected spin-readout SNR and fidelity as a function of Purcell factor and laser power. (a), (b) Expected spin state SNR for bulk (a) and surface (b) NV. (c), (d) Expected fidelity for (a) bulk and (b) surface NVs.